Fuel cells are used or have been proposed as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines and for use in stationary applications to produce electrical power. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings therein for distributing the fuel cells' gaseous reactants over the surfaces of the respective anode and cathode catalysts. A typical PEM fuel cell and its MEA are described in U.S. Pat. Nos. 5,272,017 and 5,316,871 issued respectively Dec. 21, 1993 and May 31, 1994 and assigned to General Motors Corporation.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack. Each cell within the stack comprises the MEA described earlier, and each MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluorinated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. These MEAs require certain conditions, including proper water management and humidification, for effective operation.
The electrically conductive plates sandwiching the MEAs may contain reactant flow fields for distributing the fuel cells' gaseous reactants over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels. Water (also known as product water) is generated at the cathode electrode based on the electrochemical reactions between hydrogen and oxygen occurring within the MEA. Efficient operation of the fuel cell depends on the ability to provide proper and effective water management in the system. Depending upon the operating conditions of the fuel cell stack, the water may be present in a liquid form. As the quantity of liquid water builds up, the liquid water may plug the channels through which the gaseous reactants are flowing. Additionally, the liquid water can prevent or inhibit the oxygen within the cathode flow path from traveling through the diffusion media and into contact with the catalyst layer thereby limiting the voltage production of the fuel cell stack. Thus, it would be advantageous to provide a fuel cell stack and a method of operating the same that effectively removes the liquid water formed in the fuel cells while avoiding the adverse impact on the performance of the fuel cells that can occur due to the formation of the liquid water.